Plasma welding process

Abstract: In a plasma welding process, a voltage is applied between an electrode (2) and an object for welding with injecting a plasma gas to generate a plasma (9) , and welding is performed with taking the plasma as a heat source. The process cyclically varies energy contained in a plasma arc (9) by cyclically varying a plasma gas flow rate.

Description: The present invention relates generally to a plasma welding process. More specifically, the invention relates to a plasma welding process suitable for performing plasma welding for a welding portion of a plate having a thickness of greater than or equal to 6 mm with sequentially varying welding position, such as plasma welding for medium or large diameter stationary pipe, tank or ship and so forth. A plasma welding is a welding method employing a high energy beam as a heat source. Amongst, a keyhole welding employing a plasma as the heat source has been employed in view of improvement of efficiency. Fig. 1A is a longitudinal section showing a plasma keyhole welding process, and Fig. 1B is a plan view thereof. In a welding torch 1, a non-consumable electrode 2 is arranged at the center portion. About this non-consumable electrode 2, a plasma gas nozzle 3 and a shield gas nozzle 4 are arranged in coaxial fashion. By densing energy of an arc generated by the non-consumable electrode 2 by passing through the nozzle 3 and passing a plasma gas through the high temperature arc to form a plasma state, a plasma arc 9 is generated by ionization of the plasma gas. The plasma arc 9 is cooled by the shield gas injected from the shield gas nozzle 4 to be restricted spreading and protected from oxidation by the ambient air. The plasma arc 9 is a high energy heat source locally heat a plate 5 to be welded as the base material to form a molten pool 6. On the other hand, by the plasma arc injected at high velocity, the molten metal is depressed to form a keyhole 8. By moving the plasma arc 9 along a groove 10 of the plate 5 to be welded, a plasma keyhole is advanced with melting the base material. Then, the molten metal is moved backwardly to fill the rear side keyhole. Therefore, the keyhole is constantly maintained at a fixed configuration. At the backside of the plasma arc 9 with respect to the moving direction, the molten pool 6 is formed and the molten metal is solidified to form a molten bead 7 at the further backside. The keyhole welding is a most particular high efficiency welding process of the plasma welding which can form a penetration bead as shown in Figs. 1A and 1B. Conventionally, in the plasma keyhole welding, a pulse current having a peak current value and a base current value is supplied to the electrode. The peak current value, the base current value, frequency and so forth of the pulse current are adjusted for controlling plasma arc, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. Showa 60-27473 However, this process encounters a problem in that rising period and falling period of respective pulse becomes instant upon control of the pulse current to possibly cause disturbance of the molten pool. On the other hand, the molten pool 6 has a tendency to become wide at the side of the torch 1 to be so-called wine cup like configuration. Since the width portion of the molten pool is strongly influenced by the current, higher current tends to cause widening of the width of the molten pool rather than deepening the depth of the molten pool. Increasing of the width of the molten pool will not contribute for formation of the keyhole. In case of the upward welding, the widened molten pool possibly causes melting down of the molten pool. Therefore, in the practical plasma arc control method employing the current is not so effective in prevention melting down of the molten pool and in control for all position welding, for example. In particular, in the plasma welding, the melting configuration tends to become the wine cup like configuration to have large bead width in the vicinity of the surface of the base material to cause increasing of the molten metal amount to increase tendency of drooping down or dropping down of the molten pool. This increases tendency of the phenomenon set forth above. Namely, in case of the welding for entire circumference of the medium or large diameter horizontal stationary pipe, tank or so forth, the all position welding varying the positions from downward position to upward position across vertical position has to be performed from an initial layer to a final layer. Particularly, when the plasma keyhole welding is applied for the initial layer welding, drooping down of the molten pool is easily caused. Even if the drooping down of the molten pool can be successfully prevented, there is still remained a problem to easily cause projecting bead or penetration failure. As set forth above, in the prior art, all position welding, in which the welding position is varied from the downward position to the upward position across the vertical position, should be performed over the initial layer to the final layer to make setting of the welding conditions complicate. In case of welding of the stationary pipe, particularly the horizontal stationary pipe, the welding operation is typically performed by an upwardly advancing welding progressing welding from lower portion to the upper portion or the all position welding progressing welding from downward position to upward position across the vertical position and subsequently from the upward position to the downward position across the vertical position. In such case, when the initial layer is welded by arranging the welding torch at the outside of the pipe to perform welding with directing the torch toward external surface of the pipe to be welded, the penetration bead can become excessive at the portion near the upper portion of the pipe. Conversely, lack of penetration can be caused in the vicinity of the lower portion. Therefore, it is difficult to stably form a uniform bead. In such case, while the plasma arc welding can obtain deeper penetration depth in comparison with other welding processes and thus is efficient, difficulty is encountered in stabilizing the arc. In the conventional plasma arc welding employing a pure argon gas as a plasma gas (center gas), the stability of the arc is insufficient in the case of circumferential welding of the stationary pipe. In addition, in the circumferential Welding for the stationary pipe, the plasma arc welding tends to cause a blow hole at the bottom portion of the welding bead to cause difficulty in obtaining excellent welding portion. This is particularly remarkable in the case of non-keyhole welding, in which the keyhole is not formed. In contrast to the keyhole welding, the non-keyhole welding (soft plasma welding) is employed in welding without forming the penetration bead. In such non-keyhole welding, the pure argon gas is typically used as the plasma gas. In such conventional plasma arc welding using the pure argon gas as the plasma gas, a plasma jet has a tendency to be captured in the molten pool to cause blow hole in the bottom of the bead. In the worst case, the plasma jet captured in the molten pool may form a tunnel like defect. Therefore, a difficulty is encountered in obtaining excellent welding portion. This phenomenon is remarkable in the case where the flow rate of the plasma gas is greater than or equal to 1.0 ℓ/min. On the other hand, in view of the surface property of the base material, when the base material which is processed by machining or grinding to remove surface scale and cleaned, such as a steel plate, is welded by the plasma arc welding, relatively stable and less defective molten metal can be obtained. However, when the surface is not cleaned, for example, in the case where the scale is remaining, the arc can be disturbed by the affect of the iron oxide and easily forms the defects, such as the blow hole in the molten metal. It is an object of the present invention to provide a plasma welding process which permits welding at high efficiency for the objective portion for welding where the welding position varies sequentially, without causing drooping down or dropping down of a molten pool or penetration failure, and can provide a welding portion with a deep penetration. According to one aspect of the invention, a plasma welding process, in which a voltage is applied between an electrode and an object for welding with injecting a plasma gas to generate a plasma, and welding is performed with taking the plasma as a heat source, wherein the process comprises the step of: cyclically varying energy contained in a plasma arc by cyclically varying a plasma gas flow rate. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken to be limitative to the invention, but are for explanation and understanding only. In the drawings: Figs. 1A and 1B are explanatory illustrations showing activity in a keyhole welding; Fig. 2 is an illustration showing one example of a device of pulsating a plasma gas for cyclic variation of a plasma gas flow rate; Fig. 3 is an illustration showing one example of a welding process in a circumferential welding of a stationary pipe; Fig. 4 is an illustration explaining upper end, lower end, upper half and lower half of the stationary pipe; Fig. 5 is an illustration showing another example of a welding process in a circumferential welding of a stationary pipe; Fig. 6 is an illustration showing a plasma keyhole welding process employing a backing plate; Fig. 7 is a section showing a joint configuration in the first embodiment; Fig. 8 is an illustration showing variation of the plasma gas flow rate in the first example; Fig. 9 is a section showing a joint configuration in the second embodiment; Fig. 10 is an illustration showing variation of the plasma gas flow rate in the second example; Fig. 11 is a section showing a joint configuration in the third embodiment; Fig. 12 is an illustration showing variation of the plasma gas flow rate in the third example; Fig. 13 is an illustration showing pulsation of the plasma gas flow; Fig. 14 is an illustration showing pulsation of the plasma gas flow; Fig. 15 is a section showing a manner of generation of a plasma arc; Fig. 16 is a section showing one example of a restricting nozzle configuration in the present invention; The present invention will be discussed hereinafter in detail with reference to the accompanying drawings. In contrast to the conventional plasma welding, in which a flow rate of a plasma gas is constant, the present invention cyclically varies performance of a plasma arc by cyclically varying the plasma gas flow rate. When the plasma gas flow rate is increased, a plasma state gas is injected at high energy density and high velocity to strongly depress a molten metal. Therefore, by cyclically varying the plasma gas flow rate, the performance of the plasma arc, namely penetration depth and configuration of the molten metal, can be cyclically varied. Variation of the plasma gas flow rate can be pulse like configuration of alternating sequence of a peak gas flow rate and a base gas flow rate which is smaller than the peak gas flow rate. In this case, variation period (pulse period) of the plasma gas flow rate is not particularly limited. However, when the variation period exceeds 10 Hz, the molten pool is easily disturbed to cause difficulty in obtaining healthy welding bead. Accordingly, the variation period is preferred to be less than or equal to 10 Hz, and more preferably less than or equal to 3 Hz. When the peak flow rate in a keyhole welding is less than or equal to 1 ℓ/min, the Strength of the plasma jet is insufficient to cause difficulty in performing stable keyhole welding. Also, at any thickness of plate in a non-keyhole welding, the peak flow rate greater than or equal to 3 ℓ/min will cause substantial splash of the molten metal to easily form bubble in the molten metal to result in breakage or gauging state to cause difficulty in stably obtaining good quality of welding bead. Therefore, as the peak flow rate of the plasma gas, greater than or equal to 1 ℓ/min is preferred in the case of key hole welding and less than or equal to 3 ℓ/min is preferred in the case of non-keyhole welding. The flow rate of the plasma gas in the base flow rate is preferred to be less than or equal to 3 ℓ/min in view of prevention of blowing off and melting down of the molten metal and for stability of arc during welding. Various devices may be used as a device for cyclically varying the plasma gas flow rate. For instance, the mechanism illustrated in Fig. 2 is applicable. Namely, in the conventional plasma welding apparatus, the plasma gas from a gas cylinder 11 passes a gas regulator 12 and then directly enters into a plasma welding machine 16. In the plasma welding machine 16, a hose 18 incorporating a cooling means is arranged. A welding cable is disposed through the cooling means. A welding torch 17 is connected to the plasma welding machine 16 through the hose 18. Then, the plasma gas is supplied to the welding torch 17 via the hose 18 for performing plasma welding. In the present invention, in contrast to the conventional apparatus set forth above, between the gas regulator 12 and the plasma welding machine 16 or between the plasma welding machine and the welding torch, a piping system in which piping 13 provided therewith a base gas setting needle valve 14c and a piping 13b having a peak gas setting needle valve 14a and ON/OFF electromagnetic valve 14b provided in series are connected in parallel relationship, is provided. The electromagnetic valve 14b is driven by a driving device 15 for opening and closing. The plasma welding machine 16 outputs an electric signal to the driving device 15 for controlling opening and closing of the electromagnetic valve 14b. When the electromagnetic valve 14b is driven to open by the driving device 15 in response to the electric control signal from the plasma welding machine 16, both of the plasma gas controlled the flow rate by the peak gas flow setting needle valve 14a and the plasma gas controlled the flow rate by the base gas setting needle valve 14c flow through the pipings 13b and 13a. These plasma gases join together and supplied to the plasma gas welding machine 16, and in turn into the plasma torch 17. On the other hand, when the electromagnetic valve 14b is placed in the closed position, only plasma gas passing through the base gas setting needle valve 14c is supplied to the plasma welding machine 16 and the welding torch 17. In this case, the base flow rate of the plasma gas is supplied to the welding torch 17. In a manner set forth above, the plasma gas varies the flow rate in pulse like fashion. The pulse period and the pulse height can be freely varied by controlling opening and closing of the electromagnetic valve 14b. Also, opening and closing of the electromagnetic valve 14b may be controlled by detecting an arc voltage or arc current of the plasma welding machine 16 during welding operation and by outputting the control signal from the plasma welding machine 16 to the driving device 15 in relation to the result of detection. By generating pulsating flow of the plasma gas, the welding bead with deep penetration can be attained at any welding positions without requiring large width of the bead. Needless to say, other conditions for the plasma welding will not be restricted. For instance, as the plasma gas, argon gas can be used. On the other hand, as a shield gas, argon gas, helium gas or a mixture gas mixing hydrogen or CO 2 with argon gas can be used. In case that crack due to presence of hydrogen will create a problem, the amount of hydrogen should be reduced or hydrogen is to be eliminated. The welding current, welding voltage and welding speed may be appropriately determined. In addition, additives, such as filler wire and so forth may be employed. The present invention is applicable for a plate material, such as strip, plate and so forth, pipes, tanks and so forth, even in complex configuration. Also, the invention is applicable for the materials to be welded in variety of thickness. In addition, the present invention is applicable for keyhole welding or non-keyhole welding as solely performed or in combination. Here, it should be appreciated that when a sufficient amount of welding metal can be obtained at the initial layer welding, keyhole welding is desirable. On the other hand, in case of a multi-layer welding, it is preferred to perform initial layer welding by the keyhole welding and welding for subsequent layers by the non-keyhole welding. On the other hand, helium requires higher temperature to form plasma than argon and has a property to cause a difficulty in forming the plasma jet upon initiation of arcing. However, in case of the plasma arc welding employing helium, high arc stability and good conformability between the base material and the molten pool can be attained. In addition, since helium has approximately one tenth of specific weight in comparison with argon, it can easily float on the molten metal. The inventor has found that it is possible to improve arc stability, and whereby reduce defect particularly in the bottom of the welding portion, utilizing the property of the helium, in the case of difficult welding, such as circumferential welding of the stationary pipe. As a helium gas to be employed for plasma arc welding, it should contain at least 5% to argon gas. When the content of helium gas is less than 5%, effect of arc stability and prevention of welding defect cannot be achieved. In particular, when a mixture gas formed by adding greater than or equal to 70% of helium gas to argon gas, stability of arc can be significantly improved and capturing of the plasma gas and so forth in the molten metal during welding can be successfully avoided while the penetration depth is somewhat reduced to permit production of welded metal having uniform bead configuration and no defect. When the plasma gas is formulated by combination of pure helium or helium mixture gas, the stability of plasma will not be disturbed even when the plasma gas flow rate is increased in comparison with the case where pure argon gas is used. However, when the plasma gas flow rate exceeds 15 ℓ/min, the plasma jet becomes excessively strong to cause splashing of the molten pool or capturing of the plasma gas in the molten pool. Therefore, it is necessary to limit the plasma gas flow rate to be less than or equal to 15 ℓ/min. When the plasma arc welding is to be performed for the base material which is not subject pre-treatment before welding, it is preferred to use the helium gas or a mixture gas of greater than or equal to 5% of helium gas and argon gas, as the plasma gas, and to use a mixture gas formulated by mixing one or two of CO 2 , 0 2 and H 2 to one or two of argon, N 2 and helium, as the shield gas in order to obtain deep penetration and non-defect welded portion even by the non-keyhole welding. Conventionally, pure argon gas is typically used as the shield gas, since argon is inert and inexpensive. On the other hand, helium gas has approximately one tenth of specific weight in comparison with argon, it can easily float on the molten metal. Also, nitrogen (N 2 ) is inexpensive while it is inert. Therefore, as the shield gas, argon, N 2 and helium as inert gas are used solely or as a mixture. However, when plasma arc welding is to be performed for the base material which is not subject pre-treatment before welding, it should encounter the problem as in the case of use of pure argon gas as the shield gas, in formation of defects, such as blow hole, within the molten metal. Therefore, the inventor has made extensive study in the shield gas and plasma gas and found that the problem as set forth above, can be eliminated by mixing C0 2 gas and/or O 2 gas in addition to the inert gas. This is because addition of active gas in the shield gas, the welding atmosphere is activated and stir the molten pool to float gas and/or compound harmful for health of the molten pool quickly to permit removal thereof. The effect can be equally obtained even when H 2 gas is mixed to the shield gas. In addition, H 2 is effective in enhancement of straightness of the arc. Enhancement of the arc straightness means that energy density of the plasma arc is increased by restricting the arc to increase the force of the arc. It should be noted that when cracking caused by the presence of hydrogen will cause a problem, the amount of hydrogen to be added should be reduced or hydrogen is eliminated from the mixture. When a solid wire containing one of more materials selected among Al, Ti, Zr, Si, Ni and Mn in an amount greater than or equal to 2 Wt% of the overall amount, or flux cored wire is used as a filler metal, the above mentioned effect can be further increased. When the content of these component is less than or equal to 2 Wt%, deoxidation and degassing effect can be lowered. Therefore, it is preferred that the above-mentioned component is contained in amount greater than or equal to 2 Wt%, and more preferable greater than or equal to 5 wt%. It should be noted that Al, Ti and Zr are particularly effective in fining the molten metal structure, and Ni is effective in improvement of toughness. While not specified, the following process is recommended in performing circumferential welding for the stationary pipe. In particular, the following process is particularly suitable for medium to large diameter pipes. The first process is for welding of the initial layer of the stationary pipes mating at ends in horizontal or tilted attitude. As shown in Fig. 3, when welding is to be performed along a welding line extending circumferentially, the welding torch is deposed within the stationary pipe to perform welding with taking the internal surface of the stationary pipes as the surfaces for the lower half (region X). On the other hand, for the upper half (region Y), the welding torch is arranged outside of the stationary pipe to perform welding with taking the external surface of the pipe as the surface. It should be noted that the upper half is a region including the upper end portion of the stationary pipe and the lower half is a region including the lower end portion of the stationary pipe. Accordingly, the boundary between the upper half and the lower half is not strictly limited at the center in the height, but can be at the position circumferentially shifted in the extent of 45° about the center of the stationary pipe, for example. As shown in Fig. 4, there are various ways for separating the upper half and the lower half, such as Separating at line O1 - O1, line O2 - O2, line O3 - O3 and so forth. When the boundary is set at line O2 - O2 or O3 - O3 shifted from the center in the height, i.e. line O1 - O1, the preferred range of circumferential offset is up to 45° with respect to the horizontal line extending through the center of the stationary pipe. As shown in Fig. 3, the welding torch is arranged outside of the stationary pipe to perform welding with taking the external surface of the pipe as the surface for the upper half and the welding torch is deposed within the stationary pipe to perform welding with taking the internal surface of the stationary pipes as the surfaces for the lower half. As set forth above, the position of the boundary between the upper half and the lower half is not specified to the exact center in the height of the stationary pipe, where the line extending through the intersecting points with the pipe wall and the center of the stationary pipe becomes the horizontal straight line with 180° of the center angle. The center angle of the line extending through the intersecting points with the pipe wall and the center of the stationary pipe can be differentiated from 180° depending upon the necessity. When the welding can be completed by one pass of welding operation, welding may be performed either keyhole welding or non-keyhole welding. It is also possible to simultaneously perform welding at the external surface and internal surface of the stationary pipe. In case of the multi-layer welding, it is preferred to perform all position welding for the internal surface and/or the external surface of the stationary pipe over entire circumference of the joint. Second process is a welding process for the stationary pipes mating in horizontal or tilted orientation, in which automatic welding is always started at the upper end portion and progressed downwardly. In this case, either keyhole welding or non-keyhole welding is applicable. In case of the multi-layer welding, even after initial welding, it is desirable to always perform welding from the upper end portion to the lower end portion. As set forth above, by separating the stationary pipe into the lower half (region X) to perform welding at the internal surface and the upper half (region Y) to perform welding at the external surface, the most welding position becomes downward position substantially through entire welding line with vertical position only in the limited region. As can be clear herefrom, the shown process permits to avoid the upward position in welding to contribute for speeding up and enhancing of efficiency of the welding operation. In the plasma key hole welding, stable and high efficiency welding can be achieved by using a backing material. Fig. 6 shows a plasma keyhole welding process employing the backing material. In Fig. 6, a pipe as a base material 61 is processed vertically at the mating end. A pair of base materials 61 are mated at the mating end with maintaining a small gap therebetween to define a route gap 62. The route gap 62 may be in the width of 0.5 to 5.0 mm. A backing member 63 is a steel block internally defining a cooling water passage 65. A cooling water is circulated through the passage 65 for cooling the backing member 63. The backing member 63 is formed with a slit 64 of the width greater than or equal to 3 mm at the center thereof. the backing member 63 is arranged with placing the slit 64 in alignment with the route gap 62. It should be noted that the backing member 63 is tightly fitted onto the base material 61 so as to attain good heat transmission from the base material 61 to the backing member 63. At the mating ends of the base materials, chamfer 66 of C0.5 to C3.0 is provided at the edge portions at the backside of the groove. As shown in Fig. 6, by providing the chamfer 66, the molten pool is expanded in the lateral direction, i.e. the direction along the back side of the base material. Associating therewith, the bead to be formed is widened. By expansion of the bead width, the height of the penetration bead can be suppressed to be lower than that of the case where no chamfer is provided. By this, stability of welding is increased to provide high quality welding bead. On the other hand, even in the case where the backing member is not employed, providing chamfer 66 on the edge portion at the back side of the base member to be welded as shown in Fig. 6 is effective in preventing disturbance of the plasma arc. Namely, as shown in Fig. 6, by providing the chamfer of C0.5 to C3.0 at the edge portion at the backside of the base material, effects of (1) uniform alignment of the penetration bead, (2) suppression of the penetration bead, (3) increasing of surface tension and (4) cooling effect for the penetration bead can be attained. The plasma arc passes through the pipe in wall thickness direction as a high temperature and high velocity energy pillar. However, during the period until the keyhole is formed, the plasma arc tends to be in the blocking condition. Since the plasma arc is abruptly flushed after formation of the keyhole, disturbance of the arc can be induced. By providing chamfer at the backside of the base material to be welded, disturbance of the arc at the flushing can be suppressed. Also, the velocity of the plasma arc injected at high velocity can be decelerated at the portion of the chamfer 66 to regulate the arc per se . On the other hand, the chamfer 66 is also effective for shaping and cooling the molten pool before formation of the penetration bead. In case that the chamber is not provided, the width of the penetration bead is narrow and the reinforcement (reinforcement of the penetration bead) becomes high. In contrast to this, by providing the chamfer, the width of the penetration bead is widened and height is reduced. Namely, the region to stay in the molten pool as the volume for the penetration bead becomes significantly large by providing the chamber to permit widening of the bead per se . As a result, the area to contact with the base material can be widened. By this, increasing of the molten pool cooling effect and increasing of the surface tension can be achieved to contribute for formation of good shape penetration bead. Furthermore, upon welding, by weaving the welding torch in lateral direction, stable plasma keyhole welding can be achieved even for thick base material. Namely, by weaving or swinging the welding torch in the direction perpendicular to the groove line (welding line), the plasma arc is waved. By waving of the plasma arc, deeper penetration can be achieved in comparison with the case where waving of the plasma arc is not effected, at the equal strength of the plasma arc 14. The preferred frequency of waving is 0.5 to 10 Hz, and more preferably in a range of 2 to 5 Hz. By providing waving for the plasma arc passing through the thick plate, the side wall of the base material at the side of the keyhole can be sufficiently heated to improve wettability so as to provide higher stability in keyhole welding. On the other hand, the molten pool formed at the backside of the plasma arc is widened the range to deposit on the base material in comparison with the case where no weaving is effected. As a result, improvement of the molten pool cooling efficiency and increasing of the surface tension can be achieved. As set forth above, by swinging the welding torch to cause waving of the plasma arc, thick wall welding and high speed welding becomes possible with lesser heat input. Therefore, waving of the plasma arc is effective in all position welding of the stationary pipes in a pipe line and so forth. Next, the preferred embodiments of the present invention will be discussed with comparison to comparative examples. As for these embodiments, filler wires were supplied except in the case of the first embodiment and for some examples and comparative examples in the fifth embodiment. First Embodiment This embodiment is an example for application to the all position welding for relatively small diameter pipe, having an external diameter of 216 mm. The groove was selected to be I groove (Fig. 7). One pass of welding was performed under the welding condition shown in the following table 1. As shown in Fig. 8, the peak flow rate of the plasma gas was set at 2.4 ℓ/min, the base flow rate was set at 0.6 ℓ/min so that plasma arc became strong to form the keyhole at the peak flow rate and higher solidification speed is attained for promoting formation of the bead in the base flow rate. The pulsation period of alternating the peak flow rate and the base flow rate was 1 Hz. The based material to apply the welding process was JIS G3452 200A (external diameter 216 mm, wall thickness 6 mm). Second Embodiment The shown embodiment is an example of performing one pass welding for I groove (Fig. 9) of thickness 12 mm of the base material. The welding condition is shown in the following table 2. As shown in Fig. 10, relative to 0.4 ℓ/m in of the base flow rate, the plasma gas flow rate at the peak flow rate was set at 4.2 ℓ/min to obtain strong plasma arc. The switching frequency was relatively low, i.e. 1 Hz to facilitate formation of the bead. The welding process was applied for the base material of JIS SM490 (plate thickness 12 mm). Third Embodiment This embodiment is an example, in which the keyhole welding and non-keyhole welding are selectively performed by controlling the plasma gas. The welding condition is shown in the following table 3. With respect to the pipe of 30 inch (76.2 cm) diameter, the groove is selected to be Y groove (Fig. 11), multi-layer welding was performed by performing the keyhole welding at the first pass with increased plasma gas flow rate Pmax at relatively high current side, and by performing the non-keyhole welding at the second and subsequent passes with controlling the plasma gas at lower level. The process was applied for the base material of API5L-X65 (pipe diameter 30 inch, wall thickness 19 mm). It should be noted that, in each embodiment, stable welding could be performed. The resultant bead configuration was good and no defect was found. As set forth in detail, according to the present invention, since the plasma gas flow rate is varied cyclically, the process is applicable for welding of various configuration and thickness of base materials. In particular, the welding process according to the present invention can provide deep penetration at the welding portion and high efficiency without causing drooping down of the molten pool or projecting bead. In particular, in the foregoing third embodiment, in the keyhole welding in the first pass, another welding was performed with maintaining other conditions the same as those shown in the foregoing table 3, maintaining the arc voltage at 30 ∼ 32 by the arc length control. Good quality of bead uniformly having reinforcement of weld could be obtained even at the upward position in the vicinity of the lower end of the stationary pipe while the penetration bead height was at approximately the same high to the backside of the base material in the prior art. Fourth Embodiment Under the condition shown in the following table 4, the circumferential welding was performed by way of plasma arc welding. The results of checking of workability in welding operation and defects in the welding portion are shown in the following table 5. It should be noted that among the welding process in the foregoing table, process A is a method by welding the inside of the stationary pipe for the lower half of the stationary pipe and welding the outside of the stationary pipe for the upper half thereof along the circumferentially extending welding line of the stationary pipe (Fig. 3). The B process is a method to perform automatic welding to progress welding constantly from the upper end of the stationary pipe to the lower end thereof (Fig. 5). As can be clear from the comparative examples Nos. 1, 2, 5 and 6 are examples employing pure argon gas as the plasma gas. A lot of blow holes are created in the bottom of the welded metal, possibly by capturing of the plasma gas. Also, the comparative example No. 3 had the blow hole due to too small mixture rate helium. The comparative example No.4 contained the welding defect due to excessive of plasma gas flow rate while the mixture rate of the helium was appropriate. On the other hand, the excellent welding portion having no defect could obtained in all of the examples of the present invention. Since helium or the mixture gas containing an appropriate rate of helium gas were used as the plasma gas, the plasma gas was easily float up even once captured and will not reside in the molten metal. Therefore, the defect in the bottom of the welding portion which has been the problem in the prior art, can be successfully prevented. As set out in detail, the welding process according to the present invention, can provide stable arc and excellent welding portion with no welding defect at high efficiency, particularly in the circumferential welding of horizontally oriented stationary pipe. Fifth Embodiment The plasma arc welding was performed for a steel plate under the condition indicated in the following table 6. The results of checking of the workability and defect in the welding portion are shown in the following table 7. As can be clear from the foregoing table 7, the comparative examples causes defects in the welding portion and cannot obtain the sound welded metal. On the other hand, the examples of the present invention is superior in stability of the arc and can provide sound welded metal with no defect. As set forth above, according to the present invention, when the plasma arc welding is to be performed for the base material which is not processed through preparation, the welding portion with deep penetration and no defect can be obtained even by the non-keyhole welding. As shown in Fig. 15, an injection hole 33 at the tip end of a plasma restriction nozzle 30, in which a non-consumable electrode 31, such as that made of tungsten or tungsten with rare earth metals like Th, La and Y, is disposed at the center portion, converges the plasma. The arc 32 generated from the electrode 31 forms the plasma of the plasma gas (working gas). The plasma then passes the injection opening 33 of the restriction nozzle 30 to become the high density plasma arc 32. The plasma arc 32 can be roughly separated at a portion P a at the center portion and being said to have the temperature over 10000 °C, and peripheral relatively lower temperature portion P b . In case of the plasma keyhole welding, the portion directly contributing for formation of the keyhole is the central high temperature portion P a . The peripheral portion P b mainly serves for widening the width of the molten metal by melting the portion near the surface and does not significantly contribute for penetration. Accordingly, in the keyhole welding, it is desirable to reduce the ratio of the portion P b as small as possible. Therefore, as shown in Fig. 16, is effective to provide a chamfer 34 at the injection hole 33 at the tip end of the restriction nozzle 30. The chamfer of C0.5 to 4,0 is preferred in this case. Furthermore, when a ceramic coating is provided at the tip end of the nozzle, so-called series arc phenomenon which tends to be caused when the arc length is held short to cause parallel arc from the portion other than the nozzle hole, can be successfully avoided. Moreover, overheating of the nozzle by the influence of heat radiation during welding can be also avoided. Therefore, providing ceramic coating should contribute for stabilization of the arc and for expansion of the life of the nozzle. Although the invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiment set out above but to include all possible embodiments which can be embodies within a scope encompassed and equivalents thereof with respect to the feature set out in the appended claims.

Claims: A plasma welding process, in which a voltage is applied between an electrode (2) and an object (5,7) for welding with injecting a plasma gas to generate a plasma, and welding is performed with taking the plasma as a heat source, characterized by comprising the step of: cyclically varying energy contained in a plasma arc (9) by cyclically varying a plasma gas flow rate. A plasma welding process as set forth in claim 1, characterized in that the period for varying the plasma gas flow rate is lower than or equal to 10 Hz. A plasma welding process as set forth in claim 1, characterized in that the plasma flow rate is varied in such a manner that a peak gas flow rate and a base gas flow rate are alternated sequentially. A plasma welding process as set forth in claim 3, characterized in that the peak flow rate is greater than or equal to 1 ℓ/min in case of a plasma keyhole welding and less than or equal to 3 ℓ/min in case of non-keyhole welding. A plasma welding process as set forth in claim 1, characterized in that a first piping (13a) having a base gas setting needle valve (14c) and a second piping (13b) having a peak gas setting needle valve (14a) and ON/OFF electromagnetic valve (14b) arranged in series, are provided in parallel between a gas source (11) and a welding torch (17) for varying the plasma gas flow rate by switching the electromagnetic valve ON and 0FF. A plasma welding process as set forth in claim 1, characterized in that, in circumferential welding of a stationary pipe, helium gas or a mixture gas of helium gas and argon gas containing 5% or more of helium gas is used at a flow rate of 15 ℓ/min or less. A plasma welding process as set forth in claim 1, characterized by further comprising the step of forming a shield gas flow surrounding a plasma gas flow and using a mixture gas containing one or more of argon, N 2 and helium and one or more of CO 2 , 0 2 and H 2 . A plasma welding process as set forth in claim 1, characterized by using a solid wire, consisting essentially of at least one material selected from a group of Al, Ti, Zr, Si, Ni and Mn in a content of 2 Wt% or more in total amount and the balance being iron and inevitable impurities as a filler material. A plasma welding process as set forth in claim 1, characterized by using a flux cored wire with a sheath, consisting essentially of at least one material selected from a group of Al, Ti, Zr, Si, Ni and Mn in a content of 2 Wt% or more in total amount and the balance being iron and inevitable impurities, as a filler material. A plasma welding process as set forth in claim 1, characterized in that the tip end of a nozzle for injecting the plasma is provided a chamfer of C0.5 to 4.0. A plasma welding process as set forth in claim 1, characterized in that a route gap of 0.5 to 5.0 mm is formed between the objects to be welded, and chamfer (66) of C0.5 to 3.0 is formed on the groove edge on the surfaces at the opposite sides to a plasma irradiating surface. A plasma welding process as set forth in claim 1, characterized in that an arc length is adjusted to a predetermined value by adjusting a clearance between a welding torch and the object to be welded. A plasma welding process as set forth in claim 1, characterized in that a welding torch is weaved in a direction intersecting with a welding line. A plasma welding process as set forth in claim 1, characterized in that a plasma restriction nozzle for injecting a plasma jet is provided a ceramic coating at the tip end of the nozzle.

Inventors: YOSHINO FUMITO, YOSHINO, FUMITO, YOSHINO, FUMITO, C/O FUJISAWA PLANT
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD.
Application year: 1994
Main classification: B23K10
IPC codes: B23K10, B23K33